This invention relates to an apparatus for measuring the distribution of ultrasonic wave characteristics in a medium, particularly to an apparatus utilizing the center frequency shift method for measuring ultrasonic wave attenuation constant distribution with high spatial resolution on a real time basis using simple hardware, without obtaining a reflected wave spectrum and including the ability to remove spectrum scalloping in the time domain.
The center frequency shift method is a known method for obtaining the attenuation constant in an ultrasonic wave medium. In this method, as illustrated in FIG. 1, the power spectrum A(f) of a transmitted ultrasonic wave has a Gaussian distribution expressed as EQU A(f)=Ke.sup.-[(f-f.sbsp.o.sup.).spsp.2.sup./2.sigma..spsp.2.sup.]( 1)
ln equation (1), K is a constant, the standard deviation .sigma. is a constant proportional to the bandwidth and dependent on the shape of the spectrum of the transmitted ultrasonic wave and f.sub.o is the center frequency. The power spectrum of the received signal A'(f) also has a Gaussian distribution which is expressed as follows, where f.sub.r is the center frequency of the received signal: EQU A'(f)=K'e.sup.-[(f-f.sbsp.r.sup.).spsp.2.sup./2.sigma..spsp.2.sup.]( 2)
If it is assumed that the attenuation within the medium has the form: EQU .sub.e -4f.intg..sub.o.sup.z .beta.(z)dz (2a)
where z is the distance between the transmitting/receiving transducer and the reflecting surface, f is the frequency, .beta. is the attenuation slope coefficient and "4" allows for traversing the distance z twice and conversion of pressure into power, then K' and f.sub.r are defined by the following equations: ##EQU1## Therefore, the attenuation slope coefficient .beta. can be obtained in the form indicated below by detecting the center frequency f.sub.r of the received signal. EQU .beta.(z)=(1/4.sigma..sup.2)d/dz(f.sub.o - f.sub.r) (5)
However, the received signal waveform is distorted and its spectrum deviates from a Gaussian distribution, and therefore the center frequency cannot be obtained easily. To overcome this problem, the following method has been used by the prior art to obtain the center frequency f.sub.r. The power spectrum .vertline.P(f).vertline..sup.2 is first obtained by a Fourier transformation of the time domain waveform of the received signal for a certain time window T, and then the average frequency f is obtained according to equation (6) as the first moment of the power spectrum. ##EQU2## The average frequency f is then used as the center frequency f.sub.r in equation (5) and a value for the attenuation slope coefficient .beta. is obtained using equation (5). A drawback of this method is the relatively large amount of time required for calculating the power spectrum and the first moment, and therefore, it has been difficult to obtain the distribution of .beta. on a real time basis.
In addition to deviation from a Gaussian distribution, the power spectrum of the received signal is subjected to "spectrum scalloping", which is caused by overlapping of the pulses in the received signal waveform due to reflection from random reflectors located close together in the medium. One attempt to remove spectrum scalloping in the application of ultrasonic wave detection for medical diagnosis has been to average echo envelope signals for several frequency bands. The object of this method is to remove the "speckle pattern" in the B-mode imaging. The speckle pattern is caused by interference from the spectrum scalloping and prevents the detection of small focal lesions. The speckle pattern varies with the frequency, so the averaging is effective in reducing the speckle pattern in the B-mode. This method uses linear processing and reduces the spatial resolution of the B-mode image. As a result, prior art devices using this method cannot be used for diagnosis of small focal lesions but only for diffusive lesions of a large organ such as the liver.
Spectrum scalloping can be broken into two types, slow scalloping, caused by reflection points which are very close together and rapid scalloping, caused by reflection points which are further apart. One of the inventors of the present invention developed a method for removing rapid scalloping, as diclosed in Japanese Patent Application No. 58-45396, incorporated herein by reference. This method employs a lifter in spectrum to remove rapid scallop, inverse transforms to the power spectrum and calculation of higher order moments such as M.sub.2 and M.sub.3 of the power spectrum. This method may be used to recover the true centroid from a power spectrum distorted by scallop and frequency dependent reflection. The drawback of this method is that it requires a great deal of processing in the frequency domain. If the processing is performed by hard-wired circuitry to obtain the attenuation slope coefficient .beta. as fast as possible, the cost of the equipment is extremely high. If some of the processing is performed by a programmed microcomputer, real-time calculation is not possible.
In order to avoid calculation in the frequency domain, methods have been developed, as disclosed in Japanese Patent Application Nos. 58-7726 and 58-142893, both incorporated herein by reference, which calculate the moments of the power spectrum without using Fourier transformation. In these methods, the inphase and quadrature components of the received signal waveform or the autocorrelation function of the received signal waveform are used to calculate the moments of the power spectrum. While these methods do generate the attenuation slope coefficient .beta. in real time, they do not remove the effects of spectrum scalloping.
Methods used in other fields to remove unwanted signal characteristics have been considered in attempting to solve the problems of the prior art. "Frequency agility" is used in radar systems as disclosed in Beasley, E. W. and Ward, H. R., "A Quantitative Analysis of Sea Clutter Decorrelation with Frequency Agility", IEEE. Transactions on Aerospace and Electronic Systems, Vol. 4, (1968), pps. 468-473; Gustafson, B. G. and As, B. 0., "System Properties of Jumping Frequency Radars", Philips Telecommunication Review, Vol. 25, (1964), pps. 70-76; and Ray, H. "Improving Radar Range and Angle Detection with Frequency Agility", Microwave Journal, Vol. 9, (1966), pps. 63-68. Ultrasonic flaw detection systems use "spectrum wobbling", disclosed in Koryachenko, V. D., "Statistical Processing of Flaw Detector Signals to Enhance the Signal-to-Noise Ratio Associated with Structural Reverberation Noise", Soviet Journal of Non-Destructive Testing, Vol. 11, (1975), pps. 69-75, and "split spectrum", disclosed in Newhouse, V. L., et al., "Flaw-to-Grain Echo Enhancement", Proceedings of Ultrasonics International '79, Graz, Austria, (1979), pp. 152-157 and Newhouse, V. L. and Bilgutay, N. M. et al., "Flaw Visbility Enhancement by Split-Spectrum Processing Techniques", Proceedings '81 IEEE Ultrasonics Symposium, (1981). The radar techniques vary the transmitted frequency periodically to decorrelate (randomize) the clutter echoes and use non-linear processing such as square-law-detection and averaging over the frequencies transmitted. Spectrum wobbling is a similar technique used in ultrasonic flaw detection. In the split-spectrum method, a single wideband pulse is transmitted and the received signal is separated by several filters. In both the spectrum wobbling and split-spectrum methods, the resulting signals are cross-correlated using non-linear processing and then averaged to suppress the grain echoes of the material being tested for flaws.
Thus, both the radar and flaw detection systems are designed to suppress parts of the received signals such as grain and clutter and to emphasize the "specular boundary" or shape of the object reflecting the signal. The objective of the present invention is to produce the opposite effect, that is, to suppress those parts of the signal indicating specular boundaries and to emphasize those characteristics of the signal which indicate the nature of the object reflecting the signal. In summary, no prior art method or apparatus has been disclosed which can obtain ultrasonic wave characteristics of a medium, including the attenuation slope coefficient .beta., without any spectrum scalloping in real time using low-cost equipment.